Sorry for the poor title but it is rather hard to describe my use case in a short sentence.
Context
I have the following model:
typealias Identifier = String
data class Data(val identifier: Identifier,
val data1: String,
val data2: String)
And I have three main data structures in my use case:
A Set of Identifiers that exist and are valid in a given context. Example:
val existentIdentifiers = setOf("A-1", "A-2", "B-1", "B-2", "C-1")
A Map that contains a List of Data objects per Identifier. Example:
val dataPerIdentifier: Map<Identifier, List<Data>> = mapOf(
"A-1" to listOf(Data("A-1", "Data-1-A", "Data-2-A"), Data("A-1", "Data-1-A", "Data-2-A")),
"B-1" to listOf(Data("B-1", "Data-1-B", "Data-2-B")),
"C-1" to listOf(Data("C-1", "Data-1-C", "Data-2-C"))
)
A List of Lists that group together the Identifiers that should share the same List<Data> (each List includes always 2 Identifiers). Example
val identifiersWithSameData = listOf(listOf("A-1", "A-2"), listOf("B-1", "B-2"))
Problem / Use Case
The problem that I am trying to tackle stems from the fact that dataPerIdentifier might not contain all identifiersWithSameData given that existentIdentifiers contains such missing Identifiers. I need to add those missing Identifier to dataPerIdentifier, copying the List<Data> already in there.
Example
Given the data in the Context section:
A-1=[Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A),
Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A)],
B-1=[Data(identifier=B-1, data1=Data-1-B, data2=Data-2-B)],
C-1=[Data(identifier=C-1, data1=Data-1-C, data2=Data-2-C)]
The desired outcome is to update dataPerIdentifier so that it includes:
A-1=[Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A),
Data(identifier=A-1, data1=Data-1-A, data2=Data-2-A)],
B-1=[Data(identifier=B-1, data1=Data-1-B, data2=Data-2-B)],
C-1=[Data(identifier=C-1, data1=Data-1-C, data2=Data-2-C)],
A-2=[Data(identifier=A-2, data1=Data-1-A, data2=Data-2-A),
Data(identifier=A-2, data1=Data-1-A, data2=Data-2-A)]
The reason is that existentIdentifiers contains A-2 that is missing in the initial dataPerIdentifier Map. B-2 is also missing in the initial dataPerIdentifier Map but existentIdentifiers does not contain it, so it is ignored.
Possible solution
I have already a working code (handleDataForMultipleIdentifiers() method is the one doing the heavy lifting), but it does not feel to be the cleanest or easiest to read:
fun main(args: Array<String>) {
val existentIdentifiers = setOf("A-1", "A-2", "B-1", "C-1")
val dataPerIdentifier: Map<Identifier, List<Data>> = mapOf(
"A-1" to listOf(Data("A-1", "Data-1-A", "Data-2-A"), Data("A-1", "Data-1-A", "Data-2-A")),
"B-1" to listOf(Data("B-1", "Data-1-B", "Data-2-B")),
"C-1" to listOf(Data("C-1", "Data-1-C", "Data-2-C"))
)
val identifiersWithSameData = listOf(listOf("A-1", "A-2"), listOf("B-1", "B-2"))
print("Original Data")
println(dataPerIdentifier)
print("Target Data")
println(dataPerIdentifier.handleDataForMultipleIdentifiers(identifiersWithSameData, existentIdentifiers))
}
fun Map<Identifier, List<Data>>.handleDataForMultipleIdentifiers(identifiersWithSameData: List<List<Identifier>>, existentIdentifiers: Set<Identifier>)
: Map<Identifier, List<Data>> {
val additionalDataPerIdentifier = identifiersWithSameData
.mapNotNull { identifiersList ->
val identifiersWithData = identifiersList.find { it in this.keys }
identifiersWithData?.let { it to identifiersList.minus(it).filter { it in existentIdentifiers } }
}.flatMap { (existentIdentifier, additionalIdentifiers) ->
val existentIdentifierData = this[existentIdentifier].orEmpty()
additionalIdentifiers.associateWith { identifier -> existentIdentifierData.map { it.copy(identifier = identifier) } }.entries
}.associate { it.key to it.value }
return this + additionalDataPerIdentifier
}
typealias Identifier = String
data class Data(val identifier: Identifier,
val data1: String,
val data2: String)
So my question is: how can I do this in a simpler way?
If identifiersWithSameData always contains 2 identifiers per item then it should not really be a list of lists, but rather a list of pairs or dedicated data classes. And if you convert this data structure into a map like this:
val identifiersWithSameData = mapOf("A-1" to "A-2", "A-2" to "A-1", "B-1" to "B-2", "B-2" to "B-1")
The the whole solution is pretty simple:
existentIdentifiers.associateWith {
dataPerIdentifier[it] ?: dataPerIdentifier[identifiersWithSameData[it]!!]!!
}
I'm not sure about both !!, for example I don't know if it is guaranteed that identifier existing in existentIdentifiers exists in identifiersWithSameData as well. You may need to tune this solution a little.
Related
#file:Suppress("UNCHECKED_CAST")
data class Element<T>(
val key: String,
val valueOne: T,
val valueTwo: T,
val comparator: Comparator<T>,
val comparatorValue: CompareResult
)
enum class CompareResult(
val value: Int
) {
LESS(-1),
EQUAL(0),
GREATER_THAN(1)
}
fun <T> matchesComparison(list:Collection<Element<T>>): Pair<Boolean, List<String>> {
val failedComparisons = mutableListOf<String>()
for (element in list) {
val compareValue = element.comparator.compare(element.valueOne, element.valueTwo)
if (element.comparatorValue.value != compareValue) {
failedComparisons.add(element.key)
}
}
return Pair(failedComparisons.isEmpty(), failedComparisons)
}
val stringComparator = Comparator.comparing(String::toString)
val intComparator = Comparator.comparing(Int::toInt)
val elementsToCompare = listOf(
Element("number", 1, 2, intComparator, CompareResult.LESS),
Element("first name", "a", "a", stringComparator, CompareResult.EQUAL),
Element("last name", "a", "b", stringComparator, CompareResult.EQUAL)
)
matchesComparison(elementsToCompare).second.joinToString(", ","Failed elements: \"","\"")
I often get faced with comparing two different object properties with the same values.
As an example object A has props number,firstname,lastname. What i want to do is create a list have and have a function which goes over these Elements and returns which props have failed the comparison. I've managed to use generics for both the object and the matchesComparison function which returns the failed comparisons. The problem begins when i want to pass this list which is of type Collection<Element<out Any>> to this function is i get a type missmatch. instead of using unchecked casts to force the Comparator to be of type Any i would like to do this
val stringComparator = Comparator.comparing(String::toString)
val intComparator = Comparator.comparing(Int::toInt)
The result value that of the script above should be Failed elements: "last name"
I tried changing the signature of the function to out any but then the comparator.compare method has both params as of type Nothing. I really want to avoid unsing unchecked casts.
matchesComparison() doesn't need to be generic in this case. It doesn't really care what is the type of the whole input collection, so we can simply use * here.
Then we have another problem. The compiler isn't smart enough to notice that while we perform operations on a single element, all its properties are of matching types. As a result, it doesn't allow to use element.comparator on element.valueOne and element.valueTwo. To fix this problem, we simply need to create a separate function which works on a single Element, so it understand the type for all properties is the same:
fun matchesComparison(list:Collection<Element<*>>): Pair<Boolean, List<String>> {
fun <T> Element<T>.matches() = comparatorValue.value == comparator.compare(valueOne, valueTwo)
val failedComparisons = mutableListOf<String>()
for (element in list) {
if (!element.matches()) {
failedComparisons.add(element.key)
}
}
return Pair(failedComparisons.isEmpty(), failedComparisons)
}
Also, I believe such matches() function should be actually a member function of Element. It seems strange that while Element is pretty independent and it contains everything that is needed to perform a comparison, it still requires to use external code for this. If it would have a matches() function then we wouldn't need to care about its T. matches() would work with any Element.
There is a data class as fruits.
data class Fruits(
val code: String, //Unique
val name: String
)
The base list indexed items with boolean variable is as below.
val indexList: MutableList<Boolean> = MutableList(baseFruitList.size) { false }
Now the Favourite Indexed list is as below
val favList: MutableList<Boolean> = MutableList(favFruitList.size) { true}
I want a combined full list which basically has the fav item indicated as true.
Ex:
baseFruitList = {[FT1,apple],[FT2,grapes],[FT3,banana],[FT4,mango],[FT5,pears]}
favList = {[FT2,grapes],[FT4,mango]}
The final index list should have
finalIndexed = {false,true,false,true,false}
How can we achieve in Kotlin, without iterating through each element.
You can do
val finalIndexed = baseFruitList.map { it in favList }
assuming, like #Tenfour04 is asking, that name is guaranteed to be a specific value (including matching case) for a specific code (since that combination is how a data class matches another, e.g. for checking if it's in another list)
If you can't guarantee that, this is safer:
val finalIndexed = baseFruitList.map { fruit ->
favList.any { fav.code == fruit.code }
}
but here you have to iterate over all the favs (at least until you find a match) looking to see if one has the code.
But really, if code is the unique identifier here, why not just store those in your favList?
favList = listOf("FT2", "FT4") // or a Set would be more efficient, and more correct!
val finalIndexed = baseFruitList.map { it.code in favList }
I don't know what you mean about "without iterating through each element" - if you mean without an explicit indexed for loop, then you can use these simple functions like I have here. But there's always some amount of iteration involved. Sets are always an option to help you minimise that
I have an array of customers, each customer has properties id, uuid and subCustomer and other properties that I am not interested in. I would like to do one iteration, where I would create 3 arrays where one would hold ids, other uuids and third subcustomers only.
I have tried to achieve this by using fold function like this:
customers.fold(
mapOf(
"ids" to listOf<String>(),
"uuids" to listOf<UUID>(),
"subCustomers" to listOf<String>()
))
{ acc, customer ->
acc["ids"]?.plus(customer["id"])
acc["uuids"]?.plus(customer["uuid"])
acc["subCustomers"]?.plus(customer["subCustomer"])
}
With this code I get an error in editor:
Type mismatch.
Required:
Map<String, List<{Comparable{String & UUID}> & java.io.Serializable}>>
Found:
List<Any?>?
I have tried this as well:
customers.fold(
mapOf(
"ids" to listOf<String>(),
"uuids" to listOf<UUID>(),
"subCustomers" to listOf<String>()
))
{ acc, customer ->
mapOf(
"ids" to acc["ids"]?.plus(customer["id"]),
"uuids" to acc["uuids"]?.plus(customer["uuid"]),
"subCustomers" to acc["subCustomers"]?.plus(customer["subCustomer"])
)
}
But, I get this errors:
Type mismatch.
Required:
List<{Comparable{String & UUID}> & java.io.Serializable}>
Found:
List<Any?>?
Type mismatch.
Required:
Map<String, List<{Comparable{String & UUID}> & java.io.Serializable}>>
Found:
Map<String, List<Any?>?>
Write two data classes for your data. One for your customers, and one for the three lists that you want:
data class Customer(
val id: String,
val uuid: UUID,
val subCustomer: String,
)
data class CustomerDataLists(
val ids: MutableList<String> = mutableListOf(),
val uuids: MutableList<UUID> = mutableListOf(),
val subCustomers: MutableList<String> = mutableListOf(),
)
Then, just use a simple for loop to add the data in:
val dataLists = CustomerDataLists()
for (customer in customers) {
dataLists.ids.add(customer.id)
dataLists.uuids.add(customer.uuid)
dataLists.subCustomers.add(customer.subCustomer)
}
// now dataLists is filled with customers' data
#Sweeper's answer is nice. I believe in any case it's worth using data classes instead of maps for this kind of use case.
Since you don't really have any interactions between the 3 lists in the fold, you could also build those lists independently (but it's 3 iterations of course here):
data class Customer(
val id: String,
val uuid: UUID,
val subCustomer: String,
)
data class AggregatedCustomers(
val ids: List<String>,
val uuids: List<UUID>,
val subCustomers: List<String>,
)
val customers: List<Customer> = TODO("get that list from somewhere")
val aggregated = AggregatedCustomers(
ids = customers.map { it.id }
uuids = customers.map { it.uuid }
subCustomers = customers.map { it.subCustomer }
)
This answer contiains 3 parts:
A better way to solve such problem;
Why the original code doesn't work;
Other problems need to pay attention.
1. A better way to solve such problem
Let's assume that the Consumer mentioned looks like this:
data class Customer(
val id: String,
val uuid: UUID,
val subCustomer: String,
)
It's really not necessary to use function fold in such occasion. For loop or extension function forEach is merely enough:
val customers: List<Customer> = listOf(
Customer("1", UUID.randomUUID(), "sub-1"),
Customer("2", UUID.randomUUID(), "sub-2"),
Customer("3", UUID.randomUUID(), "sub-3"),
)
val ids = mutableListOf<String>() // pay attention. use `mutableListOf` instead of `listOf()`
val uuids = mutableListOf<UUID>()
val subConsumers = mutableListOf<String>()
customers.forEach {
ids += it.id
uuids += it.uuid
subConsumers += it.subCustomer
}
2. Why the original code doesn't work
The proposed two pieces of code are in the same pattern:
customers.fold(
mapOf(
"ids" to listOf<String>(),
"uuids" to listOf<UUID>(),
"subCustomers" to listOf<String>()
)
) { acc, customer ->
// ... do something with acc and customer
}
We should first make it clear that the last statement in the fold scope is the expression to be accumulated. It's like an acc_n <combine> customer -> acc_(n+1), for each customer in customers each time, where <combine> is where we write our logic. So the first proposed piece of code doesn't work because you might not be aware that something should be returned while writing:
customers.fold(...){ acc, customer ->
acc["ids"]?.plus(customer.id)
acc["uuids"]?.plus(customer.uuid)
acc["subCustomers"]?.plus(customer.subCustomer)
}
In fact, the last statement acc["subCustomers"]?.plus(...) is an expression with type List<Any>?, kotlin regard it as your "acc_(n+1)", but you propose mapOf("ids" to ...) as acc_0, which has type Map<String, ...>: not the same type as List<Any>?. And that's why you got the first error:
Type mismatch.
Required:
Map<String, List<{Comparable{String & UUID}> & java.io.Serializable}>>
Found:
List<{Comparable{String & UUID}> & java.io.Serializable}>?
We'll talk about generic types later.
Let's move on the second piece of code. A map is proposed as the last expression in the scope of fold, which is also a map:
customers.fold(...) { acc, customer ->
mapOf(
"ids" to acc["ids"]?.plus(customer.id),
"uuids" to acc["uuids"]?.plus(customer.uuid),
"subCustomers" to acc["subCustomers"]?.plus(customer.subCustomer)
)
}
The simpliest way to eliminate error is using !! expression (not suggested!):
customers.fold(...) { acc, customer ->
mapOf(
"ids" to acc["ids"]?.plus(customer.id)!!,
"uuids" to acc["uuids"]?.plus(customer.uuid)!!,
"subCustomers" to acc["subCustomers"]?.plus(customer.subCustomer)!!
)
}
The reason is that kotlin cannot assert acc["ids"] is not null, that's why you use ?. for a null-safe method invoke. However such invoke make the return type nullable:
val cus: Customer? = Customer("1", UUID.randomUUID(), "sub-1") // cus has type Customer? : nullable
val id1: String = cus?.id // [compile error] Type mismatch. [Required: String] [Found: String?]
val id2: String? = cus?.id // OK
val id3: String = cus?.id!! // If `cus?.id` is null, throw NPE.
You've declare acc_0 (in bracket after fold) in type Map<String, List<T>> implicitly (we will talk about T later). Just know that T is not a nullable type), but a map with type Map<String, List<T>?> was found as acc_(n+1). Types mismatch and the error was shown:
Type mismatch.
Required:
List<{Comparable{String & UUID}> & java.io.Serializable}>
Found:
List<{Comparable{String & UUID}> & java.io.Serializable}>?
3. Other problem need to pay attention
An important problem is: What's the type of acc_0?
// acc_0:
mapOf(
"ids" to listOf<String>(),
"uuids" to listOf<UUID>(),
"subCustomers" to listOf<String>()
)
Of course type of each expression on the left of to is String, and List<T> is the type of each expression on the right of it. so it must be Map<String, List<T>>. What about T? Kotlin try to find the nearest ancessor of String and UUID, and find them both implements Comparable<?> and Serializable, so that's what you see in the error. That's the type of T:
Required:
List<{Comparable{String & UUID}> & java.io.Serializable}>
This may lead to some unwanted experience:
val map = mapOf(
"listA" to mutableListOf("233"),
"listB" to mutableListOf(UUID.randomUUID())
)
val listA = map["A"]!! // MutableList<out {Comparable{String & UUID}> & java.io.Serializable}!>
// generic type "collapse" into `Nothing` for no type can implement both Comparable<String> and Comparable<UUID>
listA.add(Any()) // Type mismatch. [Required: Nothing] [Found: Any]
So try not to put lists with different generic type into one map.
Another problem is, when you try to invoke acc["ids"]?.plus(customer.id), you are actually invoking such method (from kotlin _Collections.kt)
public operator fun <T> Collection<T>.plus(element: T): List<T> {
val result = ArrayList<T>(size + 1)
result.addAll(this)
result.add(element)
return result
}
A new list is created each time you invoke the method! Try use mutableListOf() in replace of listOf() for collections that you want to make changes, and use "+=" (or ?.plusAsign() as null-safe version) operator instead. This may leads to some other problem with the original code (which is too complex to explain why), but for the code in part 1: A better way to solve such problem, the += is actually invoking:
public inline operator fun <T> MutableCollection<in T>.plusAssign(element: T) {
this.add(element)
}
which just add value to list without create new ones.
I have a list of objects with an optional id as String and I want to make a map out of it.
I want to have the keys of my map as non nullable: so something like this:
data class Foo(
val id: String? = null
val someStuff: String? = null,
)
val foo = listOf(Foo("id1"), Foo())
val bar = foo.filterNot { it.id == null }.associateBy { it.id }
Here bar type is Map<String?, Foo> but not Map<String, Foo>
My workaround is to add a non null asserted call: !!, but it doesn't seem clean.
Is there an easy and safe way to do this?
This looks like something that contracts could help with, but currently a contract expression can't access properties of the class in use.
As a workaround, you could define a 2nd class that has a non-null id, like so
data class Foo(
val id: String? = null,
val someStuff: String? = null
)
data class Foo2(
val id: String,
val someStuff: String? = null
)
val foo = listOf(Foo("id1"), Foo())
val bar = foo
.mapNotNull { if (it.id != null) Foo2(it.id, it.someStuff) else null }
.associateBy { it.id }
There's a six-year-old open feature request for Map.filterNotNullKeys() and a four-year old open feature request for Map.associateByNotNull().
In my opinion, the associateBy { it.id!! } would be cleanest for readability. But you could do it like this:
val bar = foo.mapNotNull { it.id?.run { it.id to it } }.toMap()
As for your actual question, that logic is way too many steps for the compiler to infer. Your last function call to associateBy sees a nullable, so it infers a nullable. For the compiler to figure this out, it would have to step back and see that the List that you call associateBy on happens to have filtered out certain objects in a way that happens to ensure that a certain nullable property won't be null within this specific list, and it's the same property that you are associating with. Now imagine it has to do this for every call to any generic function, and the various lambdas involved could potentially have multiple lines of code. Compile times would skyrocket.
I'd like to wait until all the data from the API to be downloaded successfully and then do some operations on it.
The data result from observablesAPI1 and observablesAPI2 are different.
val observablesAPI1:List<Single<ApiResponse1>? = idApi1List.map{api1Repository.getData(it)}
val observablesAPI2:List<Single<ApiResponse2>? = idApi2List.map{api2Repository.getData(it)}
// this is not working
Single.zip(observablesAPI1,observablesAPI2,BiFunction <List<ApiResponse1>, List<ApiResponse2>> { apiResultList1, apiResultList2 -> // operations}
I thought about using nested zips but I'm not sure if it's possible to do that.
Edit:
There is actually 2 errors
when hover on observablesAPI1(similiar error on observablesAPI2):
Type missmatch. Required: SingleSource!>!
Found: List>?
when hover on BiFunction:
3 type arguments expected for fun
BiFunction(function:(t1: T1, t2: T2) ->R):BiFunction))
I would like to suggest you change the way which you map ids to data.
val observablesAPI1 = Observable.fromIterable(idApi1List)
.flatMapSingle { id -> api1Repository.getData(id) }
.toList()
val observablesAPI2 = Observable.fromIterable(idApi2List)
.flatMapSingle { id -> api2Repository.getData(id) }
.toList()
Single.zip(observablesAPI1, observablesAPI2, BiFunction<List<ApiResponse1>, List<ApiResponse2>, String> { list1, list2 ->
//do something
}).subscribe()
Note, that in my solution in zip function you have just two easy maintainable lists of ApiResponse1 and ApiResponse2.
It looks like you are having compilation errors.
This is from the javadocs of BiFunction:
/**
* A functional interface (callback) that computes a value based on multiple input values.
* #param <T1> the first value type
* #param <T2> the second value type
* #param <R> the result type
*/
public interface BiFunction<T1, T2, R> {
Your observablesAPI1 and observablesAPI2 have the type List<Single<ApiResponse*> but you are writing List<ApiResponse*>.
You are also missing the result type. For example, if you want to return a String, this is how your code should look like:
Single.zip(
observablesAPI1,
observablesAPI2,
BiFunction<List<Single<ApiResponse1>, List<Single<ApiResponse2>, String> {
apiResultList1, apiResultList2 -> "my result!"
})